1. Field of the Invention
The invention relates to warning light systems and more particularly to an improved landing approach lighting system for use in airport installations.
2. Description of the Prior Art
Red lights have been placed in the approach lighting system at airport installations as warning indicators to the pilot that the aircraft is close to the runway threshold. These lights have produced an illusion to the pilot that he is actually closer to the threshold than he actually is. This phenomenon can be responsible for landings short of the runway. Furthermore, during conditions of reduced visibility, red lights in the approach lighting system are the first part of the visible spectrum to disappear due to high absorption of the red end of the spectrum.
The most crucial area of an aircraft approach to a runway is from the last one thousand feet of altitude and three to four miles range. During this approach, the lead-in lights in the surrounding area of the runway are used by the pilot as he transitions back and forth from the instrument panel to the scene viewed through the wind screen. In most major airport installations, the lead-in lights begin at three thousand feet from the runway threshold and the aircraft at this point is normally at about two hundred and ten feet above the ground. During conditions of good visibility, the pilot usually lands from this point on by viewing the runway through the wind screen. If the aircraft is too high, the pilot may have initiated a duck under maneuver which might place the aircraft low relative to the glide slope. Because of some inherent defects in the current design of approach lighting systems, the pilot may inadvertently place the aircraft too low relative to the glide slope and consequently be forced to make a dangerous landing short of the runway threshold.
The approach lighting system of present day runways consists of signal lights arranged symmetrically about the runway center line extended starting at the landing threshold outward into the approach zone. These systems provide visual information on runway alignment, roll guidance, and horizon references. For example, in the most widely used approach lighting system, rows of red lights are placed in line with edges of the runway with added rows of white touch-down zone lights. Adjacent to the main runway and runway edge lights are taxiways which are lighted with blue lights. These blue lights are so arranged as to provide a clear and continuous indication of taxiing routes to be followed.
In the past, lighting systems have not provided adequate height perception visual cues. Some attempt has been made to place red lights on high towers. Due to chromatic aberration red lights alone produce an illusion that the obstacle is closer than it actually is.
Thus, approach lights and surrounding warning lights do not provide vertical guidance nor do they provide information regarding altitude and rate of descent. On the contrary, they can produce potentially disasterous illustions that the aircraft is too high or that its nose has pitched up. White lights contribute few visual cues and make it difficult for a pilot to determine the aircraft's real position in both altitude and range as it is approaching the runway at night. The extension of white lights out to the three thousand foot position along the runway center line contributes essentially no information other than center line guidance.
Red colored lights are placed in the approach sector of some lighting systems as warning lights to indicate to the pilot that he is over this area and short of the runway threshold. Because of the chromatic aberrations of the eye, a pilot actually perceives these lights incorrectly during a final approach.
Furthermore, during a poor visibility approach, the red lights located in the approach lighting system cannot be seen adequately. In fog, the penetration ability of red lights is very poor because of its high absorption characteristic. The first color to disappear in fog is the red end of the color spectrum.
Currently, there are no approach lighting systems which utilize the characteristics of the blue spectrum for its penetrating ability through fog.